1. Field of the Invention
The invention relates to a radio signal selective-calling receiver, and more particularly to a radio signal selective-calling receiver which is capable of detecting a difference in frequency between a base clock signal of a base station and a base clock signal of the radio signal selective-calling receiver itself in paging system, and establishing synchronization.
2. Description of the Related Art
A signal processing system for a radio signal selective-calling receiver is grouped into NTT style and POCSAG (Post Office Code Standardization Advisory Group) style.
NTT style is a perfect synchronization system in which signals are always transmitted, whereas POCSAG style is a preamble synchronization system in which signals are transmitted only when necessary. In addition, NTT style puts emphasis on enhancement of a rate at which signals are intermittently received, whereas POCSAG style puts emphasis on shortening delayed time in calling.
A lot of radio signal selective-calling receivers have been suggested so far.
For instance, Japanese Patent Publication No. 64-3413 has suggested a radio signal selective-calling receiver which stores a plurality of calling numbers through a small-sized circuitry. The suggested radio signal selective-calling receiver includes a programmable read only memory (PROM) and a decoder equipped within PROM. The radio signal selective-calling receiver attempts to reduce the number of wirings connecting PROM to the decoder, by reading out in series what is stored in PROM.
Japanese Unexamined Patent Publication No. 2-218231 has suggested a radio signal selective-calling receiver which compensates for an oscillation frequency of a local oscillator with respect to temperature. The suggested radio signal selective-calling receiver includes a device having a capacity variable in accordance with an output of a stabilized power source, in a local oscillation circuit to thereby compensate for the local oscillation circuit with respect to temperature.
Japanese Unexamined Patent Publication No. 2-504096 has suggested a data communication device resulted from improvement of a pager. The suggested data communication device is designed to include a random access memory storing broadcast messages therein and amending operation of a selected pager through broadcasting, to thereby provide flexibility to pager""s operation such as storage of data and retrieval of data.
Japanese Patent Publication No. 6-56976 has suggested a radio signal selective-calling receiver detecting signals in intermittent signal-receiving operation by counting the number of turning point signals inside and outside window signals which are in synchronization with synchronization clock signals.
The suggested radio signal selective-calling receiver is capable of stopping intermittent signal-receiving operation by subsequent signals, even though a specific pre-signal is not received, by detecting the counted number of turning point signals in the form of a signal.
A base clock signal of a base station in conventional paging system including the above-mentioned prior systems has an accuracy of xc2x1100 ppm or smaller in accordance with the standard. However, some base stations in paging system now actually in use employs a base clock signal having an accuracy in the range of 400 to 500 ppm, which is remarkably out of the standard.
If a radio signal selective-calling receiver is situated in an area covered by a base station which transmits a base clock signal with less accuracy, a call is sometimes skipped in the radio signal selective-calling receiver when calls are received, because of a difference in bit.
Hereinbelow is explained how a preamble signal is recognized in POCSAG code.
A preamble signal illustrated in FIG. 1(A) is a signal having been transmitted from a base station and demodulated by a radio signal selective-calling receiver in the case that there is no deviation in a base clock signal of a base station.
A signal illustrated in FIG. 1(B) is a synchronization clock signal for introducing data into a radio signal selective-calling receiver, and is generated based on a base clock signal transmitted from a base station. A leading edge of the synchronization clock signal is overlapped turning points of the demodulated signals illustrated in FIG. 1(A) to thereby establish synchronization in the unit of bit, and data is judged at a trailing edge of the synchronization clock signal.
Data is judged at the trailing edges of the synchronization clock signal illustrated in FIG. 1(B), namely at T1, T2, T3, T4 and T5. In the demodulated signal illustrated in FIG. 1(A), assuming that a lower level is represented with zero (0) and a high level is represented with one (1), data has a value of zero at the time T1, for instance. Hence, the demodulated signal is recognized to have values 0, 1, 0, 1 and 0 at T1, T2, T3, T4 and T5, respectively.
FIG. 1(C) illustrates a preamble signal being demodulated, which signal is obtained when a base clock signal of a base station is negatively deviated in paging system. In the demodulated signal illustrated in FIG. 1(C), assuming that a lower level is represented with zero (0) and a high level is represented with one (1), the demodulated signal has a value of zero at the time T1, for instance. Hence, if data is judged at the trailing edges of the base clock signal illustrated in FIG. 1(B), data is recognized to have values 0, 1, 0, 1 and 1 at T1, T2, T3, T4 and T5, respectively. Thus, the demodulated signal illustrated in FIG. 1(C) is not properly recognized as a preamble signal.
Japanese Unexamined Patent Publication No. 10-150682 has suggested an apparatus for making communication between a base station and a mobile station and for compensating for a local oscillation frequency of the mobile station. The apparatus is comprised of a transmitter for transmitting a calling signal from the base station to the mobile station which calling signal is comprised of a frame synchronization signal and a data signal, a receiver for receiving the calling signal at the mobile station, and a compensator for compensating for a local oscillation frequency of the mobile station in accordance with a frequency of the frame synchronization signal having been received at the receiver.
Japanese Unexamined Patent Publication No. 10-145255 has suggested a radio signal selective-calling receiver including a capacitor having a capacity variable in accordance with a voltage applied thereto, a voltage source which applies a voltage across the capacitor, and a controller controlling a voltage generated by the voltage source.
Japanese Unexamined Patent Publication No. 10-56664 has suggested a radio signal calling receiver including first means for detecting deviation in synchronization by comparing synchronization signals transmitted from a base station, to each other while a synchronization detecting window is open, and for maintaining synchronization in accordance with the detected deviation, and second means for varying a period in which the synchronization detecting window is open, in accordance with a cycle at which radio signals are to be received which cycle is assigned to a pager.
It is an object of the present invention to provide a radio signal selective-calling receiver which is capable of detecting a difference in frequency between a base clock signal transmitted from a base station and a base clock signal of the radio signal selective-calling receiver itself, and accomplishing synchronization in the unit of bit.
It is also an object of the present invention to provide a method of receiving radio signals in a radio signal selective-calling receiver which method is capable of doing the same as mentioned above.
In one aspect of the present invention, there is provided a radio signal selective-calling receiver including (a) a detector which detects a difference in frequency between a first base clock signal of the radio signal selective-calling receiver and a second base clock signal of a base station, based on third signals transmitted from the base station which is in communication with the radio signal selective-calling receiver through communication line, (b) a compensator which compensates for the first base clock signal with respect to a frequency in accordance with the detected difference, and transmits a base clock signal having been compensated for, and (c) a signal transmitter which transmits a synchronization clock signal on the basis of the base clock signal having been compensated for by the compensator.
For instance, the third signals are transmitted from the base station to establish synchronization in the unit of bit in preamble synchronization system.
It is preferable that the compensator compensates for the first base clock signal with respect to a frequency, based on a dc voltage produced in accordance with the detected difference.
It is preferable that the synchronization clock signal has the same transmission rate as a transmission rate of the third signals having been demodulated.
It is preferable that the detector is comprised of (a1) an edge detector transmitting edge signals which define leading and trailing edges of the third signals having been demodulated, (a2) a divider dividing a frequency of the first base clock signal and transmitting the thus divided base clock signal, and (a3) a phase-comparator transmitting a phase-difference signal indicative of the detected difference, based on a phase of the edge signals and a phase of the divided base clock signals.
It is preferable that the compensator is comprised of (b1) an integrator converting a phase-difference signal indicative of the detected difference, into a dc voltage, and (b2) a base clock oscillator compensating for the first base clock signal with respect to a frequency in accordance with the thus produced dc voltage.
In another aspect of the present invention, there is provided a method of receiving a radio signal in a radio signal selective-calling receiver, including the steps of (a) detecting a difference in frequency between a first base clock signal of the radio signal selective-calling receiver and a second base clock signal of a base station, based on third signals transmitted from the base station which is in communication with the radio signal selective-calling receiver through communication line, (b) compensating for the first base clock signal with respect to a frequency in accordance with the detected difference, and transmitting a base clock signal having been compensated for, and (c) transmitting a synchronization clock signal on the basis of the compensated frequency of the base clock signal.
It is preferable that the method further includes the step of producing a dc voltage in accordance with the detected difference, and wherein the first base clock signal is compensated for with respect to a frequency, based on the dc voltage.
For instance, the step (a) may be comprised of (a1) transmitting edge signals which define leading and trailing edges of the third signals having been demodulated, (a2) dividing a frequency of the first base clock signal and transmitting the thus divided base clock signal, and (a3) transmitting a phase-difference signal indicative of the detected difference, based on a phase of the edge signals and a phase of the divided base clock signals.
For instance, the step (b) may be comprised of (b1) converting a phase-difference signal indicative of the detected difference, into a dc voltage, and (b2) compensating for the first base clock signal with respect to a frequency in accordance with the thus produced dc voltage.
In still another aspect of the present invention, there is provided a data communication system including a base station and at least one radio signal selective-calling receiver such as one mentioned above, the base station transmitting third signals to establish synchronization in the unit of bit in preamble synchronization system, and being in communication with the radio signal selective-calling receiver through communication line.
In accordance with the present invention, it is possible to prevent a calling signal from being skipped, even if accuracy of a base clock signal transmitted from a base station is significantly degraded.
The above and other objects and advantageous features of the present invention will be made apparent from the following description made with reference to the accompanying drawings, in which like reference characters designate the same or similar parts throughout the drawings.